In discontinuous filter presses of this kind, the liquid component of the material for pressing is carried to the outside via filter, under the influence of a pressing pressure. The pressing pressure is exerted upon the material for pressing directly via a rigid pressure plate, or pneumatically or hydraulically via a flexible diaphragm. At the onset of supply of the material for pressing, the question arises as to what amount must be prefilled in the press chamber so that an adequate pressing cushion will be available for the first pressing. Care must be taken that with the pressure plate or diaphragm extended, the ratio between the effective filter surface area and the instantaneous press chamber volume is higher than with the press element retracted.
The press is overfilled if the ratio between the effective surface area and the instantaneous press chamber volume becomes too small as a consequence of an overly large pressing cushion, the result being worsening of the juice extraction output. In known pressing methods, the material for pressing was supplied in such a way that filling is done during a constant prefilling time chosen on the basis of empirical values. Experience has taught that for materials that are difficult to press, only a fraction of the time appropriate for readily pressed material is needed as the prefilling time. Particularly when fruit is pressed, it proves to be very difficult to ascertain an optimal prefilling time, because the pressability of the materials fluctuates very sharply from one batch to another.